Switching regulators are a common solution to provide a regulated output voltage from a varying input voltage. They are frequently implemented in battery powered electronic devices to regulate the battery output voltage which, when charged or discharged, can be greater than, less than, or substantially the same as the desired output voltage.
Conventional switching regulators generally have one or more power switches and an inductor-capacitor (LC) filter coupled between the unregulated input and the regulated output via the power switches. A control circuit selects the switching regulator configuration (i.e., the operating mode) by controlling positions of the power switches and the duration of time during which switch positions remain unchanged.
Depending on the switch configurations, the switching regulator can operate as a buck converter, a boost converter, or a buck-boost converter. A buck converter down-converts an input voltage to a lower output voltage. The power switches operating in a buck converter configuration alternately connect the input voltage to and disconnect the input voltage from the input of the LC filter. A boost converter, on the other hand, up-converts an input voltage to a higher output voltage. In the boost converter configuration, the input voltage is continuously connected to the input of the LC filter, but the inductor of the LC filter is alternately connected to and disconnected from the load where the regulated output voltage is applied. A buck-boost converter switches between the buck-converter configuration and the boost converter configuration.
During the alternate coupling performed in both the buck and boost configurations, the power switches can be exposed to large voltage spikes. These large voltage spikes can create over-voltage stress on the power switches, resulting in breakdown or reliability problems. In general, the voltage spikes are caused by parasitic inductances associated with the power lines that are coupled to and from the LC filter by the power switches.
For example, when the input of an LC filter is suddenly coupled to the input voltage in the buck converter configuration, the source providing the input voltage attempts to rapidly increase the current through the power line coupling the source and the LC filter. However, parasitic inductance associated with the power line acts to oppose any change in current by initially lowering and then oscillating the voltage, provided across the line, above and below the input voltage. Although this oscillation eventually dies down, the higher voltages caused by the oscillations, as noted above, can result in breakdown or reliability problems of the power devices.
Therefore, what is needed is a system and method for reducing voltage spikes across power devices used in voltage regulators.
The present invention will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.